1. Field of the Invention
The invention relates to methods for the preparation of concentrated aqueous bromine solutions and high activity bromine-containing solids using elemental bromine or bromine chloride. The products are used as microbiocides in water treatment.
2. Description of the Related Art
Single feed bromine biocide solutions are available from a number of sources and many methods to manufacture these products have been reported. These methods fit into two general categories: those that employ sodium hypochlorite solutions with a source of bromide ion, and those that employ elemental bromine or bromine chloride.
The prior art methods that use a sodium hypochlorite solution with a source of bromide ion yield a stabilized solution with a maximum active ingredient concentration of 14% as Br2 (6.4% as Cl2). For example, U.S. Pat. Nos. 5,683,654, 5,795,487, 5,942,126, and 6,136,205 all describe a method to manufacture a single feed, liquid bromine biocide by mixing an aqueous hypochlorite solution with bromide ion sources followed by introduction of a stabilizer agent. The method requires a complex two-vessel reaction. In the first step, NaBr and NaOCl solutions were mixed and sufficient time was allowed to permit the formation of a sodium hypobromite (NaOBr) solution. In the second step, this was then introduced to a solution of the stabilizer agent maintained at 50° C. The disadvantage of this method is that the concentration of the stabilized bromine product is limited by the concentration of NaOCl bleach that is commercially available. In fact, despite using the highest strength grade of industrial NaOCl bleach, the bromine content of the resulting stabilized liquid bromine solution was only about 14% as Br2 (6.4% as Cl2).
The prior art method that uses elemental bromine or bromine chloride yields a solution with a higher active ingredient concentration than the method that uses a sodium hypochlorite solution with a source of bromide ion. Moore, et. al. overcame the complexity of the two-vessel reaction in U.S. Pat. Nos. 6,068,861, 6,495,169, and 6,322,822 and disclosed a single-vessel reaction in which bromine or bromine chloride was added to a halogen stabilizer solution under conditions of pH control. These three patents disclosed two solutions (described in examples 4 and 5 of the '169 and '861 patents) having an active ingredient concentration of at least 19.6% as Br2 (8.7% as Cl2). However, no elevated temperature, chemical, or physical stability data was reported for either solution, and the present inventors have concluded that these two solutions are either unstable or have inferior stability.
The solution disclosed in example 5 of the '169 and '861 patents was reported to contain up to 26.7% as Br2 (11.5% as Cl2), but it had a pH of 7.0 and possessed a distinct bromine odor. It is well known that these types of solutions undergo acid-generating decomposition reactions upon storage. Thus, as the pH dropped below 7.0, highly toxic bromine vapors would have fumed from the solution and appeared as an orange/brown gas in the headspace of the container. This demonstrates that the solution of example 5 was physically unstable, and therefore, unacceptable for its intended use. The solution disclosed in example 4 of the '169 and '861 patents was reported to contain 19.6% as Br2 (8.7% as Cl2), with a pH of 13.0 and no odor of bromine. However, it has been concluded, as will be shown herein, that this solution has inferior chemical and physical stability.
Moreover, when the method using elemental bromine or bromine chloride was scaled up, as disclosed in U.S. Pat. Nos. 6,306,441, 6,352,725 and 6,348,219, the hypothetical maximum active ingredient concentration was reported to be 18% as Br2 (8% as Cl2). Subsequently, in U.S. Pat. Nos. 6,506,418, 6,511,682, and 6,652,889, Moore, et. al. reduced this ceiling even further to 14.5%-16% as Br2 (6.4-7.1% as Cl2) and required the adjustment of the pH to greater than 10 in order to produce a useful product. The '418 patent, in example 2, described the maximum strength solution as one containing 14.8% as Br2 (6.59% as Cl2).
Published application WO 03/093171 disclosed a method for preparing a stabilized bromine solution with a halogen content higher than any previously reported solution prepared from hypochlorite and sodium bromide. Example 1 of WO 03/093171 described a cumbersome multi-step method. In the first step, an unstabilized solution of sodium hypobromite was made by adding elemental bromine to a sodium hydroxide solution and allowing the mixture to react. Introducing a solution of sodium sulfamate prepared by reacting sulfamic acid with a solution of sodium hydroxide followed this. The resulting product was determined to possess a halogen content of 19.6% as Br2 (8.7% as Cl2). Example 4 of the same application disclosed an even more complex multi-step method for preparing a solution reported to have a halogen concentration of 21.6% as Br2 (9.6% as Cl2). However, it appears to the present inventors that this concentration is erroneous because the maximum concentration that can be obtained from the stated quantities of the components (even assuming a 100% yield in every step), is calculated to be 19.7% as Br2 (8.7% as Cl2). In actuality, the concentration that was obtained was probably substantially less than 19.7% as Br2 (8.7% as Cl2), because it is well-known that the first step cannot proceed with a 100% yield.
The prior art also discloses that other methods also fail to yield satisfactory products in terms of higher active ingredient concentration, physical stability, and reaction efficiency. For example, Moore described a method in U.S. Pat. Nos. 6,375,991 and 6,551,624 that used gaseous chlorine and a source of bromide ion. The reaction conditions sought the preparation of a solution possessing a theoretical bromine content of 16.8% as Br2 (7.45% as Cl2), but the actual amount reported was 10.4% as Br2 (4.6% as Cl2), corresponding to a yield of 66%. In addition, the resulting solution was stated to be physically unstable as sodium chloride salt precipitated from the aqueous phase towards the end of the reaction. Similarly, Yang, et. al. reported an identical phenomenon in U.S. Pat. No. 6,270,722.
Thus, the prior art teaches that convenient, efficient, scalable methods for preparing chemically and physically stable, liquid bromine-containing solutions will result in a product with an active ingredient concentration of 18% as Br2 (8% as Cl2) at the maximum.
Liquid bromine products that contain a higher level of active ingredient have significant economic advantages over more dilute products because a smaller amount of a more concentrated product can be used to achieve the equivalent dose of a weaker product. Also, more concentrated products need to be replaced less frequently than dilute products, and have reduced packaging, storage, and transportation costs per unit weight of active ingredient.
To be commercially viable, products with a high level of active ingredient must have two attributes. First, they must be chemically stable, i.e. they should maintain high activity for extended periods of time and not decompose quickly. Second, they must be physically stable, i.e. they must not emit dangerous fumes or precipitate or crystallize into solid salts that could plug pipe work and make the feeding of liquid materials grind to a halt.
Therefore, a need exists for methods of manufacturing liquid biocidal bromine solutions of enhanced chemical and physical stability that have a concentration of active ingredient greater than 18% as Br2 (8% as Cl2) and that conveniently employ elemental bromine or bromine chloride and a solution of sodium sulfamate. This invention addresses that need.
There is also a need for a method of producing a solid high-activity bromine-containing biocidal composition that is stable and fast dissolving. There are several solid, high-activity bromine-releasing compounds that are sold commercially as biocidal products. They are generally available as heterocyclic organic compounds to which an oxidizing bromine atom is covalently bonded to a nitrogen atom on the ring. Examples include N,N′ bromochloro-5,5-dimethylhydantoin, 1,3-dibromo-5,5-dimethylhydantoin, and mixtures of these compounds with various other components. In water, theses materials hydrolyze to release hypobromous acid, which is the biocidal agent. However, a major limitation of these solid compounds is that they are only sparingly soluble in water. Indeed, N,N′ bromochloro-5,5-dimethylhydantoin has a water solubility of only 0.1% at 20° C. As a result, bromine is released very slowly from these products as they dissolve. This is a significant disadvantage when the water requires treatment with a high, rapid dose of biocidal bromine, for example, in shock and slug dosing procedures. The low solubility of these products also precludes application where there is insufficient water available to dissolve enough of the solid to deliver a biocidally-effective dose. This invention addresses those needs.